Printed circuit boards having circuits formed on the surface of their insulating substrates are widely used in electronic parts and semiconductor devices. With recent demands for miniaturization and higher functions in electronic equipment, the printed circuit boards are required to have denser circuits and to be thinner. Establishment of a method for forming a fine circuit with a width of each line of 25 μm and a width of each space of 25 μm or lower is a particularly important task in the printed circuit board field.
As a method for producing a highly precise printed circuit board which method realizes such a fine circuit, a method called “a semi-additive method” has been proposed.
In this method, a plating catalyst such as a palladium compound is first applied to the surface of an insulating substrate made of a polymer material. Thereafter, electroless copper plating is conducted while the plating catalyst is used as a nucleus, whereby a thin copper plating film is formed on the entire surface of the insulating substrate.
Then, a photosensitive resist film (photoresist film) is applied to the surface of the thin copper layer formed by the electroless copper plating. A circuit pattern is transferred onto the resist film, and then development is carried out. Thus, a certain portion of the resist film where a circuit is to be formed is removed. Electro-copper plating is then conducted while the exposed, patterned portion of the thin copper layer is used as a power supply electrode. Thus, a copper plating film having a circuit pattern shape is formed on the surface of the thin copper layer.
Thereafter, the resist film is removed and then the thin copper layer formed by the electroless plating is removed by etching. Thus, a circuit pattern is formed. Furthermore, nickel plating or gold plating may, if desired, be conducted on the surface of the circuit pattern (the pattern formed by electro-copper plating) to produce a printed circuit board.
In the semi-additive method, a circuit pattern is formed at a circuit pitch corresponding to the resolution of a photosensitive resist film, namely development accuracy. It, therefore, enables precise formation of a finer circuit pattern than a method called “a subtractive method,” where a circuit pattern is formed by etching a thick metal foil.
However, in the semi-additive process, there is a thin copper layer formed by electroless plating between an insulating substrate and a circuit pattern. This layer, however, inherently exhibits no adhesion to the insulating substrate. Therefore, when the surface of the insulating substrate has a relatively high degree of unevenness, the adhesion between the circuit pattern and the insulating substrate is well maintained by the anchor effect of the thin copper layer. However, when the insulating substrate has a smooth surface, the adhesion is insufficient and the adhesion strength of the resulting circuit to the substrate is insufficient.
The surface of the insulating substrate is roughened for the purpose of improvement in the adhesion strength of a circuit. The surface is usually provided with unevenness having a ten-point mean roughness (Rz value), measured on the basis of JIS B0601, of approximately 3 to 5 μm. The unevenness on the surface of the insulating substrate is unlikely to cause big problems if a width of each line of the circuit to be formed is 30 μm and a width of each space of the circuit is 30 μm or more. However, when a finer pattern, for example, a circuit pattern with a width of each line 25 μm and a width of each space of 25 μm or less, is to be formed, formation of highly dense, extremely thin circuit lines is influenced by unevenness of the surface of the insulating substrate, and thus the unevenness is a serious problem.
Therefore, in forming highly precise circuit patterns such as those having a width of each line of 25 μm and a width of each space of 25 μm or less, there is a great demand for a circuit formation technique which does not cause reduction in adhesion even in the case of an insulating substrate with a high surface smoothness, for example, an insulating substrate having, as a degree of smoothness, an Rz value of 3 μm or less, and preferably 1 μm or less.
Moreover, in the semi-additive method, a thin copper layer formed by electroless plating, which layer is ultimately unnecessary, must be removed in an etching process. In the course of removal of the thin copper layer by etching, the width and the thickness of the circuit pattern composed of a copper layer formed by electoplating also decrease due to the influence of the etchant. Therefore, it is difficult to produce an accurate circuit pattern with good reproducibility. This tendency becomes more remarkable with reduction in the width and the thickness of the circuit line.
Furthermore, in the semi-additive method, a plating catalyst used in the electroless copper plating step tends to remain on the surface of the insulating substrate. Therefore, the insulating property of the resulting printed circuit board is easily lowered. In addition, when the circuit pattern is subjected to nickel plating or gold plating, which is conducted as needed after the copper plating step, nickel or gold is deposited on the surface of the insulating substrate, so that there is a possibility that an undesired circuit may be formed. Use of an etchant having a high etching ability is a conceivable solution for removing the remaining plating catalyst. However, there is a problem in that the etchant causes deterioration in reproducibility of a circuit pattern in such a case. Thus, there is a need for a method for producing a highly accurate circuit pattern with good reproducibility.